This invention relates to a semiconductor memory cell, array, architecture and device, and techniques for controlling and/or operating such cell and device; and more particularly, in one aspect, to a semiconductor dynamic random access memory (“DRAM”) cell, array, architecture and/or device wherein the memory cell includes an electrically floating body in which an electrical charge is stored.
There is a continuing trend to employ and/or fabricate advanced integrated circuits using techniques, materials and devices that improve performance, reduce leakage current and enhance overall scaling. Silicon-on-Insulator (SOI) is a material in which such devices may be fabricated on or in (hereinafter collectively “on”). Such devices are known as SOI devices and include, for example, partially depleted (PD), fully depleted (FD) devices, multiple gate devices (for example, double or triple gate), and Fin-FET. SOI devices have demonstrated improved performance (for example, speed), reduced leakage current characteristics and considerable enhancement in scaling.
One type of dynamic random access memory cell is based on, among other things, a floating body effect of SOI transistors. (See, for example, U.S. patent application Ser. No. 10/450,238, Fazan et al., filed Jun. 10, 2003 and entitled “Semiconductor Device”, hereinafter “Semiconductor Memory Device Patent Application”). In this regard, the memory cell may consist of a PD or a FD SOI transistor (or transistor formed in bulk material/substrate) on having a channel, which is disposed adjacent to the body and separated therefrom by a gate dielectric. The body region of the transistor is electrically floating in view of the insulation or non-conductive region (for example, in bulk-type material/substrate) disposed beneath the body region. The state of memory cell is determined by the concentration of charge within the body region of the SOI transistor.
With reference to FIGS. 1A, 1B and 1C, in one embodiment, semiconductor DRAM array 10 includes a plurality of memory cells 12 each consisting of transistor 14 having gate 16, body region 18, which is electrically floating, source region 20 and drain region 22. The body region 18 is disposed between source region 20 and drain region 22. Moreover, body region 18 is disposed on or above region 24, which may be an insulation region (for example, in SOI material/substrate) or non-conductive region (for example, in bulk-type material/substrate). The insulation or non-conductive region may be disposed on substrate 26.
Data is written into or read from a selected memory cell by applying suitable control signals to a selected word line(s) 28, a selected source line(s) 30 and/or a selected bit line(s) 32. In response, charge carriers are accumulated in or emitted and/or ejected from electrically floating body region 18 wherein the data states are defined by the amount of carriers within electrically floating body region 18. Notably, the entire contents of the Semiconductor Memory Device Patent Application, including, for example, the features, attributes, architectures, configurations, materials, techniques and advantages described and illustrated therein, are incorporated by reference herein.
In one embodiment, memory cell 12 of DRAM array 10 operates by accumulating in or emitting/ejecting majority carriers (electrons or holes) 34 from body region 18 of, for example, N-channel transistors. (See, FIGS. 2A and 2B). In this regard, accumulating majority carriers (in this example, “holes”) 34 in body region 18 of memory cells 12 via, for example, impact ionization near source region 20 and/or drain region 22, is representative of a logic high or “1” data state. (See, FIG. 2A). Emitting or ejecting majority carriers 30 from body region 18 via, for example, forward biasing the source/body junction and/or the drain/body junction, is representative of a logic low or “0”. (See, FIG. 2B).
Several techniques may be implemented to read the data stored in (or write the data into) memory cells 12 of DRAM device 10. For example, a current sense amplifier (not illustrated) may be employed to read the data stored in memory cells 12. In this regard, a current sense amplifier may compare the cell current to a reference current, for example, the current of a reference cell (not illustrated). From that comparison, it may be determined whether memory cell 12 contained a logic high (relatively more majority carries 34 contained within body region 18) or logic low data state (relatively less majority carries 28 contained within body region 18).
Notably, for at least the purposes of this discussion, a logic high or State “1” corresponds to an increased concentration of majority carries in the body region relative to an unprogrammed device and/or a device that is programmed with a logic low or State “0”. In contrast, a logic low or State “0” corresponds to a reduced concentration of majority carries in the body region relative to an unprogrammed device and/or a device that is programmed with a logic high or State “1”.
With that in mind, a logic high may be written into an electrically floating body transistor of a memory cell using a number of techniques. For example, a logic high may be written by impact ionization or by using a band-to-band tunneling phenomenon (hereinafter “gate induced drain leakage” or “GIDL”). Briefly, for an N-channel type SOI memory cell, a State “1” may be stored in the memory cell by creating excess holes in the electrically floating body of transistor. These holes are believed to be created by a tunneling mechanism that appears in the silicon at the edge of the drain under specific conditions. As such, where a negative voltage is applied on the gate and a positive voltage is applied on the drain, this voltage difference may create a silicon band bending that then leads to a valence band electron tunneling into the conduction band. (See, FIGS. 3A and 3B PRIOR ART). The GIDL effect or mechanism may be a very efficient manner of writing or storing a logic high (State “1”) because it tends not to cause a channel to form in the body and, as such little to no channel current flows between the source and the drain. The GIDL technique of writing or storing a logic high (State “1”) may reduce the current consumption relative to the impact ionization technique.
The TABLE 1 compares these two programming techniques or mechanisms.
TABLE 1Mechanisms used to write State “1”Band to bandChannel impact ionizationtunneling (GIDL)PowerSOI Device is ON: 10 toSOI Device is OFF:100 μA/μmlow powerScalabilityScalable for a fewMore readily scalablegenerations
Conventionally, a logic low or State “0” is written into a conventional SOI memory device while the device is in the “ON” State (for example, when the channel exists between the source and the drain). In particular, with reference to FIG. 4, conventional programming techniques for writing State “0” employ high voltage on the gate (i.e., a high gate voltage (Vg)) and a high voltage on the drain (i.e., a high drain voltage (Vd)) and, as such, the SOI memory device tends to consume and/or dissipate power (for example, approximately 200 μA/μm to approximately 800 μA/μm). Notably, State “1” is written into the SOI memory device via impact ionization.
While electrically floating body transistors of memory cells (for example, SOI transistors) of the type described above have low leakage current characteristics, such memory cells consume power when programming a logic low (i.e., removing charge carriers from the body of the SOI device). Moreover, given the need for a sufficiently large programming window (i.e., the difference in current level between a logic high and logic low), that consumption may be relatively large. As such, there is a need for high performance SOI memory cells, devices and arrays having improved performance characteristics (for example, speed and/or programming window, programming current consumption), reduced leakage current characteristics and/or considerably enhanced scaling and density capabilities.